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Atoms and Chemical Bonds
An atom is made up of protons, neutrons and electrons. Protons are positively charged and they denote the atomic number. Neutrons have no electrical charge. Protons and neutrons are present in the nucleus. Electrons are negatively charged and present in the orbits surrounding the nucleus. Isotopes have same number of protons but different number of neutrons. Loss or gain of electrons is seen in ions. Sharing electrons form covalent bond in molecules. Hydrogen bond is a type of intermolecular attraction among polar molecules such as water.

Water
Water is necessary for life because water is the medium in which all of life’s chemical reactions take place. Water is the most abundant compound in living things.

Acids, basis and buffers
Acids are electrolytes that release hydrogen ions in water. Bases are electrolytes that release hydroxyl ions in water that can combine with hydrogen ions to form water. pH represents the concentration of hydrogen ions [H+] in solution (pH = -log [H+]). Buffer solutions are solutions which resist change in pH upon addition of small amounts of acid or base. Buffers provide a relatively stable environment for biochemical reactions to take place.

Biochemical Reactions
Reactions include reactants and products. Like all other chemical reactions, biochemical reactions either release or absorb free energy. A spontaneous reaction will occur automatically whereas a nonspontaneous reaction will not occur without intervention. A spontaneous reaction releases free energy and has a negative delta G. A nonspontaneous reaction absorbs free energy and has a positive delta G.

Chemical Compounds
Chemical compounds are any substances made up of two or more atoms. Compounds join together by polymerization. Chemical compounds are identified with either their molecular (written symbols) or structural formula (arranged atoms).

Organic Molecules
Organic compounds are compounds made of carbon backbone which are the molecules found in all living things. Organic monomers join to make the four essential organic macromolecules: nucleic acids, proteins, carbohydrates, and lipids.

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An atom is made up of protons, neutrons and electrons. Isotopes have same number of protons but different number of neutrons. Loss or gain of electrons forms ion bonds, sharing electrons forms covalent bonds. Intermolecular attractions among polar molecules bridged by hydrogen are termed hydrogen bonds, which are usually weaker than ion or covalent bonds. Biochemical reactions are usually involves breaking old covalent bonds and forming new ones. Nearly all biochemical reactions are catalyzed by enzymes and occur in a buffer system which provides relative stable condition for enzymes to be active. Four major classes of macromolecules in biochemistry are polysaccharides, proteins, lipids and nucleic acids.